Composites properties are directly related to the degree of interaction between the plastic matrix and the inorganic filler. In the present work, the improvement of the composite's properties by means of the addition of surface-treated and untreated hydroxyapatite (STHA and HA, respectively) was studied. An ethylene-acrylic acid copolymer was melt blended with high-density polyethylene and HA (HDPE/HA/EA). A surface treatment was performed using an ethylene-acrylic acid (EA) copolymer for STHA(1) and acrylic acid (AA) for STHA(2). High-density polyethylene (HDPE) was also tested. STHA(1) and STHA(2) composites exhibited Young's modulus values (556 and 558 MPa, respectively) 22 % higher than that of HDPE/HA (455 MPa) and 8 % higher than that of HDPE/HA/EA (520 MPa). Additionally, STHA composites showed both yield stress and strain (sigma(STHA1)= 23 MPa; epsilon(STHA1)= 9 %; sigma(STHA2)= 22 MPa; epsilon(STHA2)= 10 %) having a remarkably different behavior from that of the HA composites, which showed no yielding at all. TEM micrographs showed better filler dispersion when surface treatment was applied to HA. Yet, the presence of EA copolymer exhibited a poorer thermal stability. The crystallinity degree as well as the crystallization and melting temperatures showed no significant variation. Regarding in vitro evaluation, composites with HA and EA copolymer proved to have better cell adhesion at early stages. The results of the STHA composites could be attributed to the electrostatic interactions taking place between the ethylene-acrylic acid copolymer and the polar groups of the HA.